As is well known in the art of aircraft control, it is desirable to control and limit the sideslip, i.e., yaw-axis motion, of an aircraft during in-flight turns and other maneuvers. Two characteristics that affect this control are the tail configuration and the implementation of pilot rudder control commands. With regard to tail configuration, the aircraft tail provides directional stability in flight and is designed to enhance other aircraft characteristics. For example, large vertical tails, sized for engine out considerations, are used conventionally for wing mounted engine configurations. With regard to pilot rudder control commands, the employment of pilot rudder control through a rudder command system has long been practiced. The rudder command system includes pilot rudder pedals, the displacement of which causes a related physical displacement of the rudder. The pilot pedal input may be proportionally modified by a rudder ratio changer. The rudder command system provides the pilot with a device for causing sideslip of the aircraft to thereby allow de-crab in crosswind landings. Additionally, the rudder command system provides a certain amount of turn coordination whereby the pilot controls sideslip in a turn. Finally, the rudder command system generally utilizes a yaw augmentation system (i.e., yaw damper) to improve Dutch roll damping characteristics. In each of the instances, the pilot may be expected to account for certain flight variables such as wind direction and speed, sideslip, yaw rate, roll angle, roll rate, lateral acceleration, etc., in determining the proper amount of rudder, i.e., pedal input, necessary to accomplish a specific maneuver.
Aircraft with wing-mounted engines and large vertical tail configurations, although benefiting from good directional stability, suffer from high structural fin loads for rudder maneuvers, high gust loads in turbulence, and poor lateral ride quality in turbulence. In contrast, aircraft having tail-mounted engines and T-tail configurations do not require a large vertical tail for engine-out considerations. In these latter configurations, the vertical tail size can be reduced. Benefits of reduced vertical tail size include: reduced gust loads and rudder maneuver structural loads; improved ride quality in turbulence; reduced weight and drag; and improved balance. One drawback from the reduced tail size is reduced directional stability.
To control an aircraft having reduced tail size, yaw augmentation is required to maintain stability and low rudder activity is required to limit fin loads. In an aircraft with reduced tail size, a closed-loop system with conventional yaw damper and turn coordination controls may be unable to maintain directional stability. Furthermore, it may be necessary to set a conventional ratio changer to a very low value (thus reducing pilot yaw control authority and ability to trim out on engine failure) to prevent the pilot from causing excessive fin load with his pedal inputs. Therefore, to improve performance in aircraft configured with reduced tail sizes, a full rudder authority closed-loop sideslip control law is beneficial in providing stability and in limiting the sideslip to allowable values set by fin loads or fin stall.
It is desirable to allow aircraft development to be taken in the direction of reduced-stability aircraft configurations by developing adequate control laws for these configurations. Otherwise, aircraft design will be limited by conventional stability criteria and the capabilities of conventional control laws. Design objectives for high-performance aircraft having reduced tail sizes include: alleviation of gust loads; reduction of tail loads; reduction of structural loads; good lateral ride quality; good Dutch roll damping; good turn coordination; automatic engine-out control in the air; and good flying qualities.
Another aspect of aircraft control is the method by which aircraft behavioral data is obtained. It is well known to use mechanical sensing devices such as accelerometers and gyroscopes for detecting inertial parameters of the aircraft. Ring-laser gyros and accelerometers are used to obtain inertial data on modern transport aircraft. Sensed air data probes, or aerodynamic measuring probes, are also used to provide data for control systems. However, there is a measurement corruption inherent in aerodynamic measuring probes caused by the local aerodynamic flow fields around the aircraft surfaces where such probes are positioned. Measurement of sideslip angle relative to the air mass is particularly problem-prone. Aerodynamic sideslip vane sensors are prone to water ingestion, icing, physical damage, and other problems. Additionally, the use of an airmass sideslip measurement as direct sideslip feedback for directional stability augmentation aggravates the ride quality problem when turbulence is encountered. Thus, a means for economically and accurately estimating a sideslip angle signal suitable for directional stability augmentation and sideslip limiting must be provided if a sideslip control law is to perform adequately.
This invention overcomes the drawbacks of existing rudder command systems as described above, as well as others.